Food Biochemistry and Food Processing

44 Part I: Principles

A reduction in the amount of phytic acid in staple
foods is likely to result in a much greater bioavail-
ability of iron and other essential minerals. Lucca et
al. (2002) inserted a fungal (Aspergillus fumigatus)
phytase cDNA into rice to increase the degradation
of phytic acid. Rice suspension cells, derived from
immature zygotic embryos, were used for biolistic
transformation with the A. fumigatusphytase gene.
Phytase from A. fumigatus was the enzyme of
choice because it is heat stable and thus can refold
into an active form after heat denaturation (Wyss et
al. 1998). The main purpose of this research was to
increase phytase activity during seed germination
and to retain the enzyme activity in the seed after
food processing and in the human digestive tract.
Although the researchers achieved high expression
levels of phytase in the rice endosperm, by placing it
under the control of the strong tissue-specific globu-
lin promoter, the thermotolerance of the transgenic
rice was not as high as expected. It has been specu-
lated that the reason for this unexpected low ther-
mostability of the A. fumigatusphytase in transgenic
rice is due to the interference of the cellular environ-
ment of the endosperm to maintain the enzyme in an
active configuration (Holm et al. 2002). Further

studies are needed to develop an endogenous phy-

tase enzyme that is thermostable and maintains high

activity in plant tissues.

ESSENTIALAMINOACIDS

Proteins are organic molecules formed by amino

acids. The digestive system breaks down proteins

into single amino acids so that they can enter into

the bloodstream. Cells then use the amino acids as

building blocks to form enzymes and structural pro-

teins. There are two types of amino acids, essential

and nonessential. Essential amino acids cannot be

synthesized by animals, including humans, and

therefore need to be acquired in the diet. The nine

essential amino acids are histidine, isoleucine, leu-

cine, lysine, methionine, phenylalanine, threonine,

tryptophan, and valine. The body can synthesize

nonessential amino acids as long as there is a proper

intake of essential amino acids and calories. Proteins

are present in foods in varying amounts; some foods

have all nine essential amino acids in them, and they

are referred to as complete proteins. Most animal

products (meat, milk, eggs) provide a good source of

complete proteins. Vegetables sources, on the other

Figure 3.7.Phytic acid is degraded during seed germination by a specific enzyme called phytase [myo-inositol-
(1,2,3,4,5,6)-hexakisphosphate phosphohydrolase] (EC 3.1.3.8).